Soil Texture Impacts Microbial Diversity

• Soil harbors high levels of microbial diversity, influencing soil functions and ecosystem services.
• The physical and chemical heterogeneity of soil is often used to explain microbial life, though a detailed understanding is still lacking.
• Soil samples were collected from 335 sites across a range of temperate oceanic habitats (e.g., arable, grassland, woodland, heathland).
• Samples were used to evaluate the link between soil texture and microbial diversity.
• Soil particle size distribution (sand, silt, clay) was measured using laser granulometry.
• Bacterial and fungal communities' diversity was determined by metabarcoding using 16S and ITS1 taxonomy marker gene regions.
• Multifractal analysis described the heterogeneity of soil particle sizes.
• Habitat type doesn't impact textural heterogeneity, and is an aspect of soil quality resistant to management decisions.
• Results indicate that soil textural heterogeneity influences microbial community diversity regardless of soil management practices and biophysical activities.
• Close linkages between soil organisms obscure the mechanisms driving biodiversity development.
• The soil physical environment impacts organisms with different life history strategies differently.
• A gram of soil can contain thousands of soil microbial taxa, but the mechanisms leading to such diversity are not well understood
• Soil particles' heterogeneity and structural arrangement may explain diversity by increasing environmental variety and isolating differentiating communities
• Soil structure heterogeneity can also lead to spatial heterogeneity of nutrient availability and physicochemical properties, which has been shown to leads to increased microbial diversity.
• Microbial communities alter the chemical environment and physical structure of soil
• Soil heterogeneity both drives and is driven by microbial diversity and function.
• Structural heterogeneity of the soil environment increases physical niche space and spatial isolation, increasing microbial diversity.
• Physical niche space is defined as the dimensions within the multidimensional ecological niche as determined by the physical environment.
• Soil microbes show preference for physical niches and differentiate between minerals and particle size fractions.
• Microbes show preference for minerals providing certain nutrients.
• The surface area to volume ratio of mineral material could influence microbial community assembly and activity.
• Differing microbial assemblages should be present as different particle size fractions become available.
• Spatial isolation of communities in soil is based on texture and water content.
• Soil water content is influenced by texture, with feedback effects on microbial communities and soil functions.
• Soil textural heterogeneity impacts microbial activity and diversity, moderated by those organisms' motility within their environment.
• Bacterial movement is largely limited to water-filled areas because bacteria have limited capacity for directed movement, only capable of moving short distances.
• Larger organisms (e.g., earthworms, plant roots) can break up the soil structure and move bacteria long distances, as can the mass flow of water.
• Hydrophobic fungi are less limited to hydrated areas and can sometimes grow across vast distances.
• Land use and management influence soil structure both directly and indirectly, impacting soil organisms and their movement.
• Organism migration and interactions change the impact of soil structure upon biological activity and diversity completely.
• Soil texture was analyzed using laser granulometry for detailed characterization of the particle size distribution.
• It was hypothesized that soil particle size bins measured by laser granulometry would correlate with other bins of a similar size.
• The influence of habitat type on soil texture class is limited, primarily due to the extensive management practices applied to the landscape, which can homogenize the soil's physical properties. Moreover, the textural class is strongly dictated by the underlying Soil texture is primarily determined by the underlying parent material, which influences mineral composition and particle size, resulting in consistent soil texture across different habitats., which determines the mineral composition and particle size, ultimately leading to consistent soil texture across varying habitats.
• Soil textural heterogeneity was expected to increase the diversity of both bacteria and fungi.
• Associations of different microbial taxa with certain particle size fractions drives diversity.
• Soil textural heterogeneity would positively impact bacterial and fungal richness after accounting for changes in pH and soil carbon (C).
• Shifts in diversity would be driven by different microbial groups associating with different particle size fractions.

Materials and methods

• Soil samples were collected as part of the Glastir Monitoring and Evaluation Programme from sites across Wales.
• Sites were randomly selected from land use classes to represent Welsh habitats and dominant soil types.
• 127 individual 1 km squares were sampled, with up to three sites randomly located within each square.
• Most sites were grassland (improved, neutral, and acid), with some arable, woodland, marshland, and other types.
• Topsoil samples (0–15 cm) were collected in summer 2013 and 2014.
• Samples were analyzed for properties including total organic C and pH.
• Soil pH was measured by suspending soil in 0.01 M CaCl2.
• Air-dried soil samples had particles >2 mm removed, and the remaining fine earth fraction was ground.
• Total organic C was measured by oxidative combustion and thermal conductivity detection.